High-resolution characterization of IGSCC crack tips and oxide films on SUS316 under simulated PWR primary water

In order to examine the influence of the boric acid concentration on IGSCC, detail analysis for the oxide film and the crack tip are performed using IGSCC specimens of clad deformed SUS316 that were tested under simulated PWR primary water with different boron concentration (1. B: 500 ppm, Li: 2 ppm DH 2.75 ppm, 320degC. 2. B: 2300 ppm, Li: 2 ppm DH 2.75 ppm, 320degC). (1) The characters of oxide films are not qualitatively different between 500 and 2300 ppm boron conditions. (2) The oxide film formed on SUS316 consists of double layers. The outer layer is mainly Fe3O4 and the inner layer is fine grain of Cr rich spinel. Moreover, enriched Ni is observed on the surface of the matrix. (3) A crack tip is filled with corrosion products that consist of chromium-rich oxide, and enriched Ni is observed ahead of the crack. This result suggests that IGSCC advance together with metal dissolution. In addition, an area of high dislocation density is observed around the crack tip. (4) The oxide film thickness formed under 500 ppm of boron was thicker than that formed under 2300 ppm of boron. (author)